1. Field of Invention
The field of the present invention relates in general to modems and more particularly digital modems.
2. Description of the Related Art
Digital Subscriber Lines (DSL) technology and improvements thereon including: G.Lite, ADSL, VDSL, HDSL all of which are broadly identified as X-DSL have been developed to increase the effective bandwidth of existing subscriber line connections to high speed back bone networks developed by telecommunications companies. An X-DSL modem operates at frequencies higher than the voice band frequencies, thus an X-DSL modem may operate simultaneously with a voice band modem or a telephone conversation. Currently there are over ten discrete XDSL standards, including: G.Lite, ADSL, VDSL, SDSL, MDSL, RADSL, HDSL, etc. Within each standard there are at least two possible line codes, or modulation protocols, discrete multi-tone (DMT) and carrier less AM/PM (CAP).
Each new XDSL protocol raises the bandwidth requirements of subscriber lines. As the bandwidth requirements increase so too does the complexity of the modem components. Additionally, because of the enormous variation in loop loss in the individual subscriber lines to which the modem may be coupled the individual components of the modem transmit and receive path must be reconfigurable to match the available bandwidth on a selected subscriber line. For modems which implement the VDSL protocol for example, the length and quality of the subscriber line will determine whether all or a portion of the two upstream and two downstream communications ranges will be available. The sheer length of the line results in significant attenuation of the upper communication ranges rendering their use impractical. Even on shorter loops the presence of bridges and taps can also constrain bandwidth. In each instance it is critical during the line qualification phase to be able to inexpensively and quickly determine insertion power loss into the subscriber line to which the modem is coupled.
What is needed is a modem with improved capabilities for determining insertion loss into a subscriber line.
The current invention provides a method and apparatus for determining insertion loss in wireline communication systems. The method may be implemented with any modulation protocol but is particularly suited for multi-carrier modulation protocols such as discrete multi-tone (DMT).
The method for calculation of the insertion loss uses a received signal and a discrete subset of frequency ranges, e.g. DMT tones, within that signal. For each of the subset of frequency ranges the average received power on each of the selected set of tones is determined. Next the average power on each received subset of tones is converted to log2 using a simple log2 estimation circuit that avoids the complexity associated with log tables and other prior art methods of computing logarithms. Then the sum of the log2 of the average power received in each of the selected subset of tone bins is determined. Next, the known transmitted power for the selected subset of tones is expressed as log2, is summed and is then subtracted from the sum of the log2 of the average power received. Then using a simple electrical length estimation circuit the insertion loss is calculated. A corresponding apparatus is disclosed.